This invention relates generally to down hole tools and methods for manufacturing such items. More particularly, this invention relates to infiltrated matrix drilling products including, but not limited to, polycrystalline diamond compact (“PDC”) drill bits, natural diamond drill bits, thermally stable polycrystalline (“TSP”) drill bits, bi-center bits, core bits, and matrix bodied reamers and stabilizers, and the methods of manufacturing such items.
Full hole tungsten carbide matrix drill bits for oilfield applications have been manufactured and used in drilling since at least as early as the 1940's. FIG. 1 shows a cross-sectional view of a down hole tool casting assembly 100 in accordance with the prior art. The down hole tool casting assembly 100 consists of a thick-walled mold 110, a stalk 120, one or more nozzle displacements 122, a blank 124, a funnel 140, and a binder pot 150. The down hole tool casting assembly 100 is used to fabricate a casting (not shown) of a down hole tool.
According to a typical casting method as shown in FIG. 1, the thick-walled mold 110 is fabricated with a precisely machined interior surface 112, and forms a mold volume 114 located within the interior of the thick-walled mold 110. The thick-walled mold 110 is made from sand, hard carbon graphite, or ceramic. The precisely machined interior surface 112 has a shape that is a negative of what will become the facial features of the eventual bit face. The precisely machined interior surface 112 is milled and dressed to form the proper contours of the finished bit. Various types of cutters (not shown), known to persons of ordinary skill in the art, can be placed along the locations of the cutting edges of the bit and can also be optionally placed along the gage area of the bit. These cutters can be placed during the bit fabrication process or after the bit has been fabricated via brazing or other methods known to persons of ordinary skill in the art.
Once the thick-walled mold 110 is fabricated, displacements are placed at least partially within the mold volume 114 of the thick-walled mold 110. The displacements are typically fabricated from clay, sand, graphite, or ceramic. These displacements consist of the center stalk 120 and the at least one nozzle displacement 122. The center stalk 120 is positioned substantially within the center of the thick-walled mold 110 and suspended a desired distance from the bottom of the thick-walled mold's 110 interior surface 112. The nozzle displacements 122 are positioned within the thick-walled mold 110 and extend from the center stalk 120 to the bottom of the thick-walled mold's 110 interior surface 112. The center stalk 120 and the nozzle displacements 122 are later removed from the eventual drill bit casting so that drilling fluid can flow through the center of the finished bit during the drill bit's operation.
The blank 124 is a cylindrical steel casting mandrel that is centrally suspended at least partially within the thick-walled mold 110 and around the center stalk 120. The blank 124 is positioned a predetermined distance down in the thick-walled mold 110. According to the prior art, the distance between the outer surface of the blank 124 and the interior surface 112 of the thick-walled mold 110 is typically 12 millimeters (“mm”) or more so that potential cracking of the thick-walled mold 110 is reduced during the casting process.
Once the displacements 120, 122 and the blank 124 have been positioned within the thick-walled mold 110, tungsten carbide powder 130 is loaded into the thick-walled mold 110 so that it fills a portion of the mold volume 114 that is around the lower portion of the blank 124, between the inner surfaces of the blank 124 and the outer surfaces of the center stalk 120, and between the nozzle displacements 122. Shoulder powder 134 is loaded on top of the tungsten carbide powder 130 in an area located at both the area outside of the blank 124 and the area between the blank 124 and the center stalk 120. The shoulder powder 134 is made of tungsten powder. This shoulder powder 134 acts to blend the casting to the steel and is machinable. Once the tungsten carbide powder 130 and the shoulder powder 134 are loaded into the thick-walled mold 110, the thick-walled mold 110 is typically vibrated to improve the compaction of the tungsten carbide powder 130 and the shoulder powder 134. Although the thick-walled mold 110 is vibrated after the tungsten carbide powder 130 and the shoulder powder 134 are loaded into the thick-walled mold 110, the vibration of the thick-walled mold 110 can be done as an intermediate step before the shoulder powder 134 is loaded on top of the tungsten carbide powder 130.
The funnel 140 is a graphite cylinder that forms a funnel volume 144 therein. The funnel 140 is coupled to the top portion of the thick-walled mold 110. A recess 142 is formed at the interior edge of the funnel 140, which facilitates the funnel 140 coupling to the upper portion of the thick-walled mold 110. Typically, the inside diameter of the thick-walled mold 110 is similar to the inside diameter of the funnel 140 once the funnel 140 and the thick-walled mold 110 are coupled together.
The binder pot 150 is a cylinder having a base 156 with an opening 158 located at the base 156, which extends through the base 156. The binder pot 150 also forms a binder pot volume 154 therein for holding a binder material 160. The binder pot 150 is coupled to the top portion of the funnel 140 via a recess 152 that is formed at the exterior edge of the binder pot 150. This recess 152 facilitates the binder pot 150 coupling to the upper portion of the funnel 140. Once the down hole tool casting assembly 100 has been assembled, a predetermined amount of binder material 160 is loaded into the binder pot volume 154. The typical binder material 160 is a copper alloy.
The down hole tool casting assembly 100 is placed within a furnace (not shown). The binder material 160 melts and flows into the tungsten carbide powder 130 through the opening 158 of the binder pot 150. In the furnace, the molten binder material 160 infiltrates the tungsten carbide powder 130. During this process, a substantial amount of binder material 160 is used so that it fills at least a substantial portion of the funnel volume 144. This excess binder material 160 in the funnel volume 144 supplies a downward force on the tungsten carbide powder 130 and the shoulder powder 134. Once the binder material 160 completely infiltrates the tungsten carbide powder 130, the down hole tool casting assembly 100 is pulled from the furnace and is controllably cooled. The thick-walled mold 110 is broken away from the casting. The casting then undergoes finishing steps which are known to persons of ordinary skill in the art, including the addition of a threaded connection (not shown) coupled to the top portion of the blank 124 and the removal of the binder material 160 that filled at least a substantial portion of the funnel volume 144. Typically, this binder material 160 is not reusable because metallurgical bonds are formed between the binder material 160 and the blank 124 and is not very pure to allow the binder material 160 to be reused. At today's pricing, the binder material 160 is approximately seven dollars per pound. Significant cost reductions can be made if an economical method is found for maintaining the purity of the excess binder material and reusing at least a portion of the excess binder material 160 that filled at least a substantial portion of the funnel volume 144.
Hard carbon graphite is typically used in making the thick-walled mold 110 because it is easily machinable to tight tolerances, conducts furnace heat well, is dimensionally stable at casting temperatures, and provides for a smooth surface finish on the casting. However, a primary drawback in using a hard carbon graphite mold 110 is that it has a lower thermal expansion rate than the steel blank 124 that is disposed within the mold 110 to form the casting around it. As a result of this difference in thermal expansion rate, the diameter of the steel blank 124 is decreased and the diameter of the mold 110 is increased to constrain the forces that are generated during the casting process. These differences in thermal expansion rate between the steel blank 124 and the hard carbon graphite mold 110 create a risk that the graphite mold 110 will crack, thereby destroying the casting.
The primary reason for mold cracking lies in the dissimilarity of the coefficient of thermal expansion of three major components of the down hole tool casting assembly 100. These major components are the steel blank 124, the tungsten carbide powder 130, and the graphite mold 110. The blank 124 has a relatively high coefficient of thermal expansion, while the tungsten carbide powder 130 and the graphite mold 110 have extremely low coefficients of thermal expansion. When the down hole tool casting assembly 100 is heated in a furnace, the outside diameter of the blank 124 expands as the temperature increases, thereby putting pressure on the densely packed tungsten carbide powder 130. The tungsten carbide powder 130 transmits this pressure to the internal diameter of the graphite mold 110, thereby creating hoop stress. If the wall of the graphite mold 110 is too thin, then the hoop stress overcomes the strength of the graphite mold 110 and a crack occurs which leads to the molten binder material 160 leaking through the graphite mold 110, a scrapped casting, and other consequential damages. These consequential damages include loss of material, increased labor costs, missed delivery, very expensive damage to the furnace, and loss of production for several days.
According to one example in the prior art, a twelve and one-fourth inch drill bit casting is typically fabricated using an eighteen inch diameter graphite mold 110 even though the twelve and one-fourth inch drill bit casting physically can be made using a fourteen inch diameter graphite mold 110. The extra four inches in diameter provides a safety factor against the mold 110 from cracking. This safety factor comes at a substantial cost because larger diameters of graphite molds 110 increase in cost per diameter inch along a steeply ascending slope. FIG. 2 shows a graph 200 illustrating the relationship between total graphite diameter 210 versus cost 220. A linear inch of fourteen inch diameter graphite costs approximately fifty dollars, while a linear inch of eighteen inch diameter graphite costs approximately seventy-five dollars. A ten inch tall mold of fourteen inch diameter graphite will have a graphite cost of approximately five hundred dollars, while a ten inch tall mold of eighteen inch diameter graphite will have a graphite cost of seven hundred and fifty dollars. Thus, a significant cost savings can be made in the fabrication of the mold 110 if the safety factor became unnecessary or reduced.
In the prior art, a further step that has been used to mitigate cracking of the graphite mold is to use a smaller diameter blank 124 to reduce hoop stress pressure developed during heating in the furnace. However, this step increases the cost of fabricating the casting because additional expensive tungsten carbide powder 130 is required to fill the mold. At today's pricing, the blank 124 costs approximately fifty cents per pound, while the tungsten carbide powder 130 costs approximately twenty-five dollars per pound. Thus, a significant cost savings can be made in the fabrication of the casting if larger diameter blanks 124 can be used without increasing the risk of cracking the graphite mold 110.
In the prior art, the increased costs associated with fabricating a casting has been tolerated by manufacturers because of the risks and costs associated with mold 110 failure.
In view of the foregoing discussion, need is apparent in the art for improving the casting process so that the costs associated with casting fabrication are decreased. Additionally, a need is apparent for improving the casting process so that some of the costs associated with mold failure are mitigated. Further, a need is apparent for improving the casting process so that a significant portion of the binder material is reusable. Furthermore, a need is apparent for improving the casting process so that a smaller diameter mold is used in the casting process. Moreover, a need is apparent for improving the casting and the casting process so that a smaller volume of tungsten carbide powder is used in the casting process. A technology addressing one or more such needs, or some other related shortcoming in the field, would benefit down hole drilling, for example fabricating castings more effectively and more profitably. This technology is included within the current invention.